The field of the invention generally relates to a method of bending tubes such as for making heat exchangers, and more particularly relates to a method of splitting a bend die to reduce vertical spacings between adjacent parallel segments of a tube heat exchanger.
As is well known, residential furnaces have been constructed using tubular heat exchangers instead of the more conventional clam-shell heat exchangers. With such arrangement, a plurality of stainless steel or aluminized steel tubes are arranged within a heat exchange chamber of a furnace and one end of each is fired by an individual burner. The hot combustion gases pass through the tubes, and heat is transferred to household return air that is forced across outside surfaces of the tubes.
In the above-described furnace arrangement, it is desirable to maximize the heat exchange surface area within the confined or restricted volume inside the heat exchange chamber. It may also be desirable to minimize the size, and in particular, the height of the heat exchange chamber so that the furnace can be used at installations that have height restrictions. For example, a furnace 40 inches high can be sold into markets where 48 inch furnaces will not fit. Accordingly, tubes have been bent into serpentine configurations with parallel straight segments to increase the length of tubes that will fit into a heat exchange chamber. In particular, tubes have been rotated between successive bends so that the parallel straight segments are not linearly aligned. Therefore, when the parallel segments are viewed from their ends, the bends can be seen to zigzag back and forth. The zigzagging is desirable because it promotes turbulence in the return air that is forced across the outside surfaces of the tubes. Thus, heat transfer is enhanced.
Another reason for zigzagging relates to the apparatus used to bend the tubes. In particular, one apparatus is described in U.S. Pat. No. 5,142,895. A tube is seated in the groove of a rotary bend die, and a pressure die and clamp die are moved up against the opposite side of the tube. The bend die and the clamp die are then rotated approximately 180 degrees about a vertical axis while the pressure die moves forward linearly carrying the tube tangentially to the bend point. The clamp die and pressure die are then retracted and returned to their respective initial positions, and the tube is repositioned with respect to the bend die so that another 180 degree bend can be executed. The tube may also be rotated to elevate the just formed segment above the path used by the clamp die on the next bend. This tube rotation leads to segments that zigzag rather than being disposed in a single plane. The apparatus further had a split bend die wherein an upper section was elevated from a lower section to remove the tube which had been formed with controlled wrinkles past the 180 degree tangent point.
One drawback of the above described system was that the height between adjacent horizontal segments was limited to certain minimums because the tube rotation angle or zigzag angle was restricted to certain minimums. More specifically, as the tube was moved forward to reposition it for the next bend, the angle between successive bends or segments had to be relatively large such as 108 degrees to clear the upper section of the bend die. Raising the upper section of the bend die aided by making the angle less restrictive, but the angle was still larger than desirable for certain applications. For a given set of conditions such as tube diameters, horizontal spacings between burners, and center line radii for bends, the relatively large angle between successive bends resulted in relatively large vertical spacings between successive segments. Therefore, for a given low profile furnace, the segments could not be packed densely enough to attain a desired heat transfer characteristic.